1. Field of the Invention
The present invention relates to a ceramic die for use in cutting and shaping a lead frame with a semiconductor chip mounted thereon in an assembling process for semiconductor devices. More specifically, the present invention relates to a ceramic die for use in cutting a series of tie bars, trimming and/or punching an edge of the lead, and/or bending the lead.
2. Description of the Prior Art
Semiconductor devices are typically assembled through the following process. A semiconductor chip is secured to a lead frame or a package, following which electrodes of the device are interconnected through wires to leads of the lead frame. The frame or the package is then sealed with a resin. Tie bars connecting a plurality of the leads of the lead frames and the lead frames are cut after sealing. Further, cutting and punching of edges of the leads and bending of the leads are performed.
In this assembling process, cutting of the tie bars, cutting and punching of the edges of the leads and bending of the leads are conducted by using a cutting and shaping die. Such a cutting and shaping die is disclosed in, for example, Japanese Patent Laid Open No. 5-82689. The cutting and shaping die comprises a die set and punches with a comb-shaped working section. Conventional cutting and shaping dies for lead frames are typically made of hardened steel or cemented carbide. These materials are subjected to electric discharge machining, following which the materials are subjected to polishing and surface finishing by polishing to form a working section.
However, the dies made of such metal material have a disadvantage dispersion in working precision due to deposition and/or adhesion of debris generated in working the lead frame when cutting and punching soft metal leads consisting mainly of Cu or Al.
In addition, the leads of the lead frame are arranged at a smaller pitch in recent years. The comb-shaped working section of the die thus has teeth with a smaller gap. The die is required to have improved precise working characteristics. Accordingly, there has been a demand on a die material having a high Young's modulus and a low plastic deformation property to inhibit or prevent plastic deformation represented by warping after shaping and other deformation of the material, such as burr, occurring during shaping and trimming. The die is also required to have a high hardness and a superior wear resistance to reduce wearing of the die.
On the other hand, a solder plating used to fix the semiconductor device to a printed wiring board may cause a problem in the conventional die made of the metal material. More specifically, the solder plating may adhere to a surface of the working section of the die. Continuous working of the lead frames with the solder adhered to the surface of the die may cause deformation of the lead such as bending. Such deformation may be a potential cause of inferior soldering, inferior contact, or short-circuit of pins in the subsequent processes. Thus, there may arise problems such that a final product of the semiconductor device operates incorrectly.
With this respect, the die is removed from the machine regularly to remove the adhered solder. In this event, the solder is lapped and then removed. A specific method of cleaning the die is disclosed in, for example, Japanese Patent Laid Open No. 4-280500, in which a brush cleaning or air-blow cleaning mechanism is provided. This mechanism is cooperated with a moving mechanism for the product during bending of an outer lead of the lead frame to remove forcedly the solder adhered to the die. This method using mechanical removal of the solder adhered to the die requires to ensure a sufficient space to attach the mechanism or even modification of the die. The die may be deformed and a lifetime of the die may be reduced.
Japanese Patent Laid Open No. 2-28964 discloses a method of inhibiting adhesion of the solder debris by means of forming grooves in portions of a surface of the bending punch or the bending die where the lead frame contacts. A cost for the die is increased if the grooves are formed in all portions where the solder debris is expected to adhere. In addition, it is difficult to remove the solder debris deposited within the grooves. This makes it necessary to improve the die by the maintenance considerations.
Japanese Patent Laid Open No. 2-134856 discloses a method of depositing a film of amorphous chromium plating having a thickness of from 5 to 10 .mu.m and having superior wear resistance and sliding properties as well as non-adhesion properties on the surface contacting with the lead frame of the die. The film exhibits a weak adhesion to a substrate. Another problem of this method is a rough surface of the film appeared just after formation of the film.
On the other hand, Japanese Patent Nos. 63-203222 and 203223 disclose to use plasma CVD to deposit a film of, for example, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, diamond, or SiC on a surface of a die made of steel or cemented carbide for working metals. Alternatively, Japanese Patent Laid Open No. 64-62468 discloses a method of depositing a hard film of diamond or diamond-like carbon having a thickness of from 0.1 to 10 .mu.m on the surface of a metal or ceramic die, by using the plasma CVD or the like.
However, these films have low adhesion to the substrate and chipping may be caused when a die has a complex configuration and is used under a severe working condition as in working the lead frame. As a result, the lifetime of the die is shortened. In addition, the diamond film has a rough surface. The surface roughness should be improved by other working when this film is used for a die, which disadvantageously takes a long time.